1. Field of the Invention
The present invention relates to a PSK (Phase Shift Keying) signal demodulation method as well as a device for realizing the method, and in particular, to a PSK signal modulation method and device for cases such as satellite communication that require both highly stable demodulation operation as well as high-speed synchronized pull-in characteristics over a broad frequency range under low C/N (Carrier-to-Noise ratio) conditions.
2. Description of the Related Art
FIG. 1 is a block diagram showing a PSK signal demodulator of the prior art. An inputted N-phase PSK signal (N-PSK signal) R(t) is first N-frequency multiplied at N-frequency multiplier 1 to eliminate the modulation component, and a signal having a frequency N times the carrier frequency is outputted from N-frequency multiplier 1. Adaptive line enhancer 2 has the characteristic of a narrow-band band pass filter adapted to the line of the signal of the output of N-frequency multiplier 1, and accordingly, when a line signal is inputted, adaptive line enhancer 2 allows its frequency component to pass. The noise component from the output of N-frequency multiplier 1 is eliminated from the output of adaptive line enhancer 2, and the output therefore becomes a signal in which the unmodulated signal, i.e., the line component, is emphasized. Phase detector 3 generates phase signal .theta..sub.A which corresponds to the phase angle of the output of adaptive line enhancer 2. Differentiator 4 differentiates phase signal .theta..sub.A, and converts it to angular frequency signal .omega..sub.N. Multiplier 5 multiplies angular frequency signal .omega..sub.N by 1/N and generates recovered carrier angular frequency .omega..sub.R. Integrator 7 integrates recovered carrier angular frequency .omega..sub.R and calculates phase angle .theta..sub.S of the recovered carrier. Phase angle .theta..sub.S is then converted to a -sin function value by -sin converter 8 and a cos function value by cos converter 9.
The operation of the circuit from phase detector 3 to multiplier 5 corresponds to the N frequency division of the frequency, and accordingly, the outputs of -sin converter 8 and cos converter 9 can be used as a recovered carrier for demodulation.
Complex multiplier 10 multiplies the complex-indicated outputs of -sin converter 8 and cos converter 9 by complex-indicated input signal R(t) and generates demodulated signal D(t).
FIG. 2 is a signal waveform diagram illustrating the operation of each circuit component of the PSK signal demodulation circuit shown in FIG. 1. The PSK signal demodulator shown in FIG. 1 operates as follows:
Following receptions inputted N-phase PSK signal R(t) is an asynchronous quadrature detected (i.e., quasi-coherent detected) signals and accordingly, is made up of an in-phase component series and a quadrature-phase component series.
The quasi-coherent detected N-phase PSK signal R(t) can be represented by a complex number as shown in equation (1) below: EQU R(t)=exp j{(2.pi.k(t)/N)+.theta.(t)}!+z(t) (1)
where z(t) is the noise component, k(t) is an integer 0.ltoreq.k&lt;N, and .theta.(t) is phase error.
When this signal is N-frequency multiplied at N-frequency multiplier 1, the output M(t) becomes the unmodulated signal as shown in equation (2): EQU M(t)=R(t).sup.N =exp jN.theta.(t)!+z'(t) (2)
The output of adaptive line enhancer 2 becomes signal A(t) in which noise component z'(t) is eliminated from M(t) as shown in equation (3): EQU A(t)=exp jN.theta.(t)! (3)
Phase detector 3 outputs signal .theta..sub.A (t), which corresponds to the phase of signal A(t): EQU .theta.A(t)=N.theta.(t) (4)
After this signal is time-differentiated at differentiator 4, the signal .omega..sub.R (t) multiplied by 1/N at multiplier 5 is as shown in equation (5): EQU .omega..sub.R (t)=d.theta.(t)/dt (5)
The output of multiplier 5 is outputted to integrator 7. The output .theta..sub.S (t) of integrator 7 is as shown in equation (6), from which it can be seen that the carrier phase is recovered: EQU .theta..sub.S (t)=.intg..omega..sub.R (t) dt (6)
The output .theta..sub.S (t) of integrator 7 is inputted to -sin converter 8 and cos converter 9. If the output of cos converter is represented as a real part and the output of -sin converter is represented as an imaginary part, the output C(t) of -sin converter 8 and cos converter 9 are as shown in equation (7): EQU C(t)=cos .theta.(t)-j sin .theta.(t)=exp -j.theta.(t)! (7)
Multiplying this result by input signal R(t) at complex multiplier 10 produces the result shown in equation (8): EQU D(t)=R(t)C(t)=exp j2.pi.k(t)/N!+z(t) exp -j.theta.(t)! (8)
A phase-synchronized demodulated signal can be obtained.
Because adaptive line enhancer 2 has not yet adapted to input immediately following the start of input of signal R(t), output does not accord with the output shown in equation (3) and the output of complex multiplier 10 is not phase-synchronized, but adaptive line enhancer 2 adapts to input quickly and phase synchronization is established.
Due to the employment of an adaptive line enhancer, the PSK signal modulation device of the above-described prior art offers the advantage of high-speed demodulation synchronization capabilities even in cases in which the frequency offset of the input signal is large. However, the use of N-frequency multiplication to effect carrier recovery carries with it the drawback of degradation of the signal-to-noise ratio of the input signal to the adaptive line enhancer.
To improve the signal-to-noise ratio of the recovered carrier, the bandwidth of the band-pass filter formed at the adaptive line enhancer must be restricted. As is known in the art, the number of taps of the FIR Finite Impulse Response! adaptive filter must be increased to narrow the bandwidth of the adaptive line enhancer, but this increase brings about a considerable increase in the circuit scale of the FIR adaptive filter. Thus, improvement of the signal-to-noise ratio of the carrier entails greatly increased circuit scale. On the other hand, limiting the scale of the circuit brings about a drop in the signal-to-noise ratio of the carrier as well as the occurrence of cycle slips and so forth in the demodulated signal, and stable demodulation operation therefore cannot be obtained
Thus, while a PSK signal demodulation device of the prior art employs an adaptive line enhancer to obtain a high-speed demodulation synchronization pull-in characteristic, such a construction necessitates an extremely large-scale circuit if stable demodulation operation is to be attempted after demodulation synchronization.